Edwin S. Robinson

Ocean Tide and Waves Beneath the Ross Ice Shelf, Antarctica

The ocean tide in the southern Ross Sea is principally diurnal. The tropic tide range (double amplitude) is between 1 and 2 meters, depending on the location, and is closely related to the local water-layer thickness. The range of the tropic tide is more than three times the range of the equatorial tide. Cotidal and coamplitude charts were made for the largest diurnal constituents, K1 and O1 and a provisional cotidal map was made for the semidiurnal constituent M2. The amplitudes of the diurnal tide constituents are larger in the Ross Sea than in the adjacent southern Pacific Ocean, indicating the existence of a diurnal resonance related to the shape and depth of the sea. Waves related to ocean swell propagate into the ice-covered region from the northern Ross Sea. These waves have amplitudes near 1 centimeter, and periods in the range 1 to 15 minutes. The speed at which these waves travel is successfully predicted by flexural wave theory.

Mechanical Disintegration of the Navajo Sandstone in Zion Canyon, Utah

Three groups of features including columns and column recesses, projecting plates, and arches result from mechanical disintegration of sandstone cliffs in Zion Canyon, Utah. The disintegration is in part due to frost action, runoff from rain showers, seep and stream undercutting, and gravity stresses in the rock masses. Rock slides from the canyon walls appear to occur randomly. Estimates of the quantity of material that could be released by failure of suspended columns and projecting plates were computed from elastic considerations and values of 20 kgm/cm 2 and 700 kgm/cm 2 taken to be representative of the tensile and crushing strengths of the sandstone. Arches may originate from seep undercutting or intersection of joints paralleling the canyon wall; both of these factors contribute to the formation of vertical plates subjected to tensile stresses. Examination of simple mechanical models suggests that arches may develop upward as the result of rock failure along curved stress trajectories.

Correlation of tidal gravity and heat flow in eastern North America

Abstract An apparent correlation of tidal gravity and heat flow can be examined using data from ten sites in eastern North America. A trend is clearly evident on the plot of M2 tidal gravity residuals and estimated heat flow values with a linear regression slope of 0.043 μgal m 2 mW −1 and a multiple correlation coefficient of 0.79. The mean deviation of points from the linear regression line is 0.2 μgal. Similar plots for the O1, K1, and S2 harmonics reveal positively sloping trends, but multiple correlation coefficients smaller than 0.06 indicate that little significance can be attached to these apparent trends. The O1, K1, and S2 tidal gravity residuals, as well as the N2 residuals, are not determined accurately enough to verify the trend found for the stronger M2 harmonic. The well-known linear relationship between heat flow and heat production within the eastern North American heat flow province has been interpreted to indicate significant upper crustal influence on spatial variations in heat flow. Therefore, a correlation of tidal gravity with heat flow implies a measurable upper crustal tidal response.

Geophysical evidence for an allochthonous Alleghanian(?) granitoid beneath the basement surface of the Coastal Plain near Lumberton, North Carolina

Geophysical data obtained over the Atlantic Coastal Plain near Lumberton, North Carolina, indicate the presence of a granitic pluton buried beneath basement volcanic rocks similar to those of the Carolina slate belt. The volcanic rocks were cored southeast of the city of Lumberton. They consist of interlayered felsic and mafic rocks metamorphosed to lower amphibolite facies. Excellent reflections from a nearby VIBROSEIS line originate from this volcanic sequence, which is interpreted as being >3.5 km in thickness. The volcanic rocks are underlain by an acoustically transparent interval interpreted as being associated with a granitic pluton. This hypothesis is supported by a −35 mgal Bou-guer gravity anomaly and by a relatively high heat flow of 63.5 ± 5.4 mW/m 2 obtained in the drill hole. These are both characteristics of late Paleozoic Alleghanian granitoids in the southeastern United States. Results of gravity modeling suggest that the body is nearly circular in shape (∼45 km diam) and 13 km in thickness. Subhorizontal reflections at 5 to 7 sec on the seismic data are interpreted as being at and below the base of the granitoid at a depth of ∼ 17 km. The deep layering may be stratigraphy or mylonites, and, in either case, the granitoid is allochthonous and did not intrude the layering. High heat flow suggests, from our experience in the Piedmont, that the body is unmetamorphosed and of late Paleozoic age; the thrusting may postdate, or be coeval with, the age of intrusion and suggests that late Paleozoic thrusting in this part of the Coastal Plain is confined to depths below 17 km.

The Use Of Poisson's Relation For The Extraction Of Pseudototal Magnetic Field Intensity From Gravity Observations

Investigation of geological structure by gravimetric and magnetic field surveys requires consideration of relationships between gravity anomaly and magnetic anomaly generating sources. The possibility of using Poisson’s Relation to examine magnetic and gravity fields related to a common source is intriguing. This relation is expressed as follows: A(x,y,z)=IGρ∂U(x,y,z)∂α, (1) where A (x, y, z) is the magnetic field potential and U (x, y, z) is the gravity field potential at a point in space due to a source of uniform density ρ and uniform magnetization I in the direction α. This expression has been used to derive magnetic anomalies over idealized forms (Nettleton, 1940) and, by Baranov (1957), to extract pseudogravity fields from magnetic field data. The purpose of this paper is to develop an expression for extracting a pseudomagnetic field from gravity field data and to examine the practical applications of this expression.

Static corrections on the southeastern Piedmont of the United States

On the Piedmont of the southeastern United States, seismic reflection statics at different points in a typical common‐depth‐point (CDP) gather can differ from one another by more than 50 msec because of topographic relief in excess of 50 m, variations in the thickness of the weathered zone that can extend deeper than 50 m, and variation of velocity in the weathered zone of more than 100 m/sec. The ABCD method for computing statics is introduced to account for these velocity and thickness variations as well as the topographic effect. This method combines elevations and positions of source‐receiver points with times of first arriving refracted waves read from reflection correlograms or seismograms. It was tested in central Virginia where typical piedmont conditions are encountered. At four locations, ABCD statics are close to control values determined independently from refraction experiments. At 99 source‐detector points, ABCD statics differ by an average of 4 msec, (maximum of 19 milliseconds) from conven...

UPWARD CONTINUATION OF TOTAL INTENSITY MAGNETIC FIELDS

Upward continuation of gravity and magnetic field data has been used in interpretation for over two decades. The additional perspective gained from viewing a field at different levels has been widely recognized as useful in interpreting subsurface anomaly sources.

Relations of Bouguer Gravity Anomalies to Geological Structure in the New River District of Virginia

Over 500 gravity observations were made at 2- to 3-km intervals along accessible roads in the New River District of Virginia, within an area defined by lat 37°009N., and 37°309N., and long 80°159W. and 81° 009W. The Bouguer anomalies of the data can be related to variation in crustal thickness, and folding and faulting in the Appalachian Valley. A quadratic polynomial surface fitted to Bouguer anomaly data by a least squares process reveals a broad anomaly trending north-northeasterly across the area. This anomaly correlates well with the gravity field associated with variation in crustal thickness determined from a seismic survey of the Middle Atlantic states. Residual gravity anomalies obtained from differences between the Bouguer anomalies and the quadratic polynomial surface indicate an east-northeasterly trending feature which parallels the regional strike of the Appalachian geological structure. This residual gravity anomaly of approximately 8 mgal can be explained by a structural model based upon attitudes of sedimentary units measured at the surface and density values measured from outcrop specimens. This interpretation suggests that the Blacksburg syncline consists of approximately 7 km of Paleozoic sedimentary rocks. There is no indication in the gravity data of any major decollement. Numerous local residual anomalies 2 to 3 mgal in amplitude are observed over the Pulaski and Saltville thrust faults. These anomalies may be associated with the extensive breccia zones exposed in the near surface material and with the variation in the thick-nesses of thrust plates.